Lunar orbiters reveal a world shaped by impacts

Largest of these probably punctured the Moon's crust.

If you're spending the money to put something in orbit around the Moon, you'll likely stuff it full of as many scientific instruments as it can fit. But now NASA has sent not one, but two spacecraft to the Moon with only a single scientific instrument. Despite the light load, the GRAIL mission turns out to have been a phenomenal success, as it's providing the clearest map of the composition of our closest neighbor. The results show a world shaped by a combination of impacts (some of which probably penetrated to the Moon's mantle) and the stretch marks of an early expansion in its size.

The GRAIL mission was modeled on NASA's earlier GRACE satellites, which were used to study the Earth. The satellite's lone instrument keeps track of the distance between the orbiting pair, which changes due to the gravitational attraction caused the terrain below. Thanks to the Moon's lack of significant atmosphere and small gravitational draw, the twin GRAIL probes were able to orbit at an average of 55km above the surface, providing details nearly three times better than any previous mapping effort.

The initial mapping mission started in March 2011 and ran for a bit over a year. It was able to map differences with a resolution of about 13km, and it obtained more than 99.99 percent of all the data it possibly could, given the resolution of its instruments. This is likely to be the clearest picture we have of the Moon for a while.

As far as the Moon is concerned, what we see is what is there. According to one of the three papers on the mission released by Science today, more than 98 percent of the local variations in gravitational attraction are the product of the surface topography. In other words, the craters and ridges that we see on the Moon's surface produce the majority of the signal picked up by GRAIL. That's uniquely high among the bodies we've looked at. The Earth, Venus, Mars, and Mercury all have much greater internal variability, in most cases the product of tectonic activity.

Although the Moon experienced some volcanic eruptions, the majority of the surface features come from impacts. In the map shown on top, these impacts leap out with high densities in the center (where material was compacted and heated) surrounded by areas of shattered, lower-density material. In fact, so many impacts have affected the crust that the material is, on average, fairly porous and relatively homogeneous. In other words, the impacts acted a bit like a giant food processor. The GRAIL data also indicates that it is probably thinner than had been predicted.

That thickness is rather significant. "With a thinner crust," the authors note, "it becomes increasingly probable that several of the largest impact events excavated through the entire crustal column and into the mantle." Their modeling of the crust suggests two impact regions have an interior thickness of roughly zero (Moscoviense and Crisium), while three others are near zero (Humboldtianum, Apollo, and Poincaré).

One of the three papers performed an analysis where the signal from the obvious surface features was eliminated. This revealed the remaining two percent, caused by internal features hidden beneath the Moon's surface. The most obvious features in the remainder were long lines, some of them extending for nearly a thousand kilometers. These features were relatively deep, starting at least 5km down and extending down to at least 70km. And they were old, as large impact craters from early in the Moon's history interrupted them.

The authors suggest the features are analogous to "dike swarms" on Earth, where tectonic rifts allowed deeper, molten material to invade the crust. Although the Moon never had much in the way of plate tectonics, it is thought the heat left over from the impact that created it created a global magma ocean underneath the crust. That would provide the molten material, but how did the fracture occur?

The authors note models of the early Moon suggest its layered structure included a relatively cool interior, molten ocean, and cooler crust. This structure should simultaneously warm the interior while cooling the outer shell, leading to an overall net expansion of the Moon. The entire radius of the Moon is predicted to have grown by somewhere between 0.6 and 4.9 kilometers within its first billion years, then later contracted. The authors think that would be sufficient to create large-scale cracks in the lunar crust, which the magma ocean then filled.

Overall, the GRAIL results tell us a lot about the early history of the Moon, and they put some constraints on models of its formation. In addition, they provide some hints about the environment of the inner Solar System shortly after its formation by telling us something about the impacts that all the bodies must have been experiencing, even though other processes gradual erased that evidence. Not bad for a single scientific instrument.

Sometimes less is best, at least for certain applications. NASA once again shows itself to be a master of the probe/satellite method of investigation. I use gravity maps in my geology work, so it's really interesting to see what they look like on the Moon and that, unlike Earth, almost all of the gravity anomalies are essentially surface dictated.

Give it a rest, science hater. This is non-trivial stuff. As Timmer says, Earth has sub-surface density variations that result in measurable gravitational differences. A few milli-Gals may not seem like much, but for satellites in low orbit it can add up to major shifts over time.

Also, it suggests that the moon's geology is highly uniform, so if/when we start lunar mining we know what to expect.

@mattclary, this probe was looking for gravity anomalies, not magnetic ones. Sadly, other missions have already found that TMA-1 does not exist in this universe.

Now that this process has ended. They should drop the orbit to 10km height and try and map as much as they can. OK, it'l take a lot more time, but there should be a real significant increase in resolution.

I am very excited by the GRAIL findings and its implication for early Earth history.

- The high degree of crustal porosity ("an average crustal porosity of 12% to depths of at least a few kilometers") and the correlated "role of impact bombardment in homogenizing the distribution of shallow density anomalies" seems to confirm the heavy bombardment and, presumably, its long tail.

-- If so, we didn't have to (wait for a) return to the Moon to untangle the Apollo material datings!

-- If so, life arose amidst or likely before the heavy bombardment, survivable even for mesophiles by cell proliferation and dispersal in models, but now perhaps firmly tested at an even higher impact rate than earlier believed.

- I hear the estimated crust/mantle aluminum content, presumably from the mentioned constraint on other elements is similar to Earth's. This is yet another successful test of the Earth-Moon impactor. ("the bulk refractory element composition of the Moon is not required to be enriched with respect to that of Earth")

- The thinner than earlier estimated crust promises to mean a large and long active mantle/core akin to Earth's, yet another observation of the E-M impactor event. ("an average crustal thickness between 34 and 43 km").

- The dichotomy between nearside crustal thickness (~20 km) and the farside thickness (~60 km), shown in GRAIL data video releases, promises to elucidate Moon history and inner state further.